The present invention relates to a raw material composition for soda-lime glass, and more particularly to a raw material composition for soda-lime glass capable of effectively suppressing formation of nickel sulfide (NiS) in a glass base in the course of melting of the glass raw material, to thereby produce a glass product of high quality.
In a conventional method for producing soda-lime glass, in a step for melting glass raw material at a temperature as high as near 1,500xc2x0 C. in a melting furnace, a nickel (Ni) component contained in stainless steel used for the interior of the melting furnace and Ni-containing metal particles (e.g., stainless steel particles) present in glass raw material as an impurity may be mixed into molten glass, and the Ni component may react with a sulfur (S) component in mirabilite (Na2SO4) serving as a glass raw material. As a result, nickel sulfide (NiS) may be present as a fine impurity in a melt-molded glass substrate. The incidence of an NiS impurity in a defective glass product is very low; i.e., the number of impurities is about one in some 10 tons (t) of glass products. In addition, the impurity has a spherical particle and the particle size is as small as 0.3 mm or less, and thus detection of the impurity in a production line is very difficult.
In order to process a substrate formed of such soda-lime glass into glass for a building or a toughened glass plate for an automobile, the substrate is heated to the softening point (near 600xc2x0 C.) and quenched, to thereby produce compressive stress in the surface layers of the glass plate.
When nickel sulfide (NiS) is contained as an impurity in toughened glass which is heated and cooled to ambient temperature in a toughening step, xcex1-phase NiS, which is stable at about 350xc2x0 C. or higher, is present in an unstable state. Since xcex1-Phase NiS is unstable at ambient temperature, with passage of time it is transformed into xcex2-phase NiS, which is stable at ambient temperature. The volume of NiS increases concomitant with phase transformation. A toughened glass plate contains a tensile stress layer having a thickness which is about ⅔ the overall thickness of the plate, and thus cracks grow rapidly due to an increase in NiS volume in the tensile stress layer, to thereby cause spontaneous breakage of the glass plate.
In order to prevent such spontaneous breakage of toughened glass, a method for removing a defective product containing an NiS impurity is known (which method is called soaking treatment). In the method, toughened glass which is heated and cooled to ambient temperature in a toughening step is placed in a firing furnace (a soaking furnace) and re-heated and maintained therein for a predetermined period of time, and any unstable xcex1-phase NiS contained in the toughened glass is transformed into xcex2-phase NiS, which is stable at about 300xc2x0 C. or less, to thereby increase the volume of NiS and compulsorily break the defective glass.
However, in such steps mainly comprising thermal treatment, a long period time and a great amount of thermal energy are used in order to raise temperature, and thus production cost may increase. In addition, such steps raise a serious problem against reduction in production time and enhancement of productivity.
In order to solve the aforementioned problems involved in conventional techniques, an object of the present invention is to provide a raw material composition for soda-lime glass capable of effectively suppressing formation of nickel sulfide (NiS) in the course of melting of the glass raw material.
Another object of the present invention is to provide a raw material composition of soda-lime glass capable of effectively suppressing formation of NiS in the course of melting of the glass raw material when the material contains, as a coloring component, ferric oxide (Fe2O3), selenium (Se), cerium (Ce), or other metallic materials in a very small amount.